A secondary battery is a battery that can be repetitively recharged. The charging characteristics of a secondary battery including the behavior of the battery terminal voltage as charging progresses often depends upon the type of the secondary battery. For example, a nickel-cadmium battery has a charging characteristics in which the terminal voltage of the secondary battery typically peaks when the battery is fully charged. On the other hand, nonaqueous secondary batteries, such as, for example, a nonaqueous, organic electrolyte, lithium secondary battery (hereinafter referred to as a lithium ion secondary battery), usually does not exhibit a peak of the terminal voltage. Instead, the terminal voltage usually continues to increase as the battery is overcharged. The latter charging characteristics of nonaqueous secondary batteries can make detection of the fully charged state more difficult.
In addition, when secondary batteries of any type are overcharged, the properties of the secondary batteries are usually deteriorated. For example, the life span of the secondary battery is typically decreased by an overcharge. A lithium ion secondary battery is particularly susceptible to weakening as a result of excessive charging.
To prevent the lithium ion secondary battery from being overcharged to an excessively high voltage, U.S. Pat. No. 5,028,860 describes a method of charging using a constant voltage. When the lithium ion secondary battery is charged with a constant voltage, the charging current typically decreases, as the charging progresses to the full charge level. Accordingly, the charging current may be monitored, and when the charging current becomes smaller than a predetermined current value, the charging current can be shut off.
Charging with a constant voltage usually prevents the secondary battery from being overcharged. This is a result of the fact that the terminal voltage of the secondary battery typically does not exceed the applied constant voltage of the charger. However, even though the charging apparatus outputs a constant output voltage, the effective voltage (V.sub.off) at which the secondary battery is actually being charged is different from the constant output voltage (V.sub.out ) which the charging apparatus applies across the battery. For example, FIG. 1 shows an equivalent circuit of a typical secondary battery which is set between output terminals A, B of a charging apparatus. The battery usually has a series loss resistance (R) which comprises the internal resistance of the secondary battery and the contact resistances between the output terminals A, B and terminals of the secondary battery. Furthermore, when lead wires are connected between the output terminals A, B and the terminals of the secondary battery, the loss resistance can also be increased by the lead wire resistances. Moreover, the loss resistance of the battery typically is not fixed. For example, the internal resistance of the battery can increase as the secondary battery ages, or as the ambient temperature falls. In addition, the contact resistances can increase as a result of poor contacting conditions.
During charging, as long as the charging current (I) flows, a voltage drop (V.sub.drop =R.times.I) is caused by the loss resistance (R). Therefore, even though the charging apparatus outputs a constant output voltage (V.sub.out), the secondary battery is actually charged with an effective voltage (V.sub.off =V.sub.out -V.sub.drop) different from the applied voltage V.sub.out. Because both the loss resistance R and the charging current can vary as set forth above, this effective voltage is not constant.
For example, when the charging apparatus outputs a constant output voltage (V.sub.out =4.10 V) and the voltage drop (V.sub.drop) caused by the loss resistance (R) is 0.10 V, the secondary battery is actually being charged with an effective real voltage (V.sub.off =4.00 V) which is less than the applied voltage (V.sub.out). One consequence of the effective voltage (V.sub.off) being smaller than the applied constant voltage is that it can take longer to fully charge the secondary battery. On the other hand, if the constant output voltage (V.sub.out) is set to a higher value of 4.20 V, for example, to compensate for the loss resistance to shorten the charging time, the battery can become overcharged. More specifically, as the battery becomes fully charged, the charging current approaches zero and therefore the voltage drop (V.sub.drop) also approaches zero. Consequently, the effective charging voltage (V.sub.off) approaches the applied charging voltage (V.sub.out) and the secondary battery is actually overcharged with an overvoltage of the applied constant output voltage (V.sub.out =4.20 V). Overcharging is especially troublesome for lithium ion secondary batteries which can become weakened as a result of the overvoltage.
It is seen from the above that it is difficult to select a suitable constant voltage value for the output charging voltage (V.sub.out). For example, if the constant voltage value of the output voltage (V.sub.out) is set to a high value, the secondary battery may be overcharged with an overvoltage at the fully charged state. On the other hand, if the constant voltage value of the output charging voltage (V.sub.out) is set to a low value, the secondary battery may not become fully charged or it may take too long to fully charge the secondary battery.
In general, a charging method which uses a constant voltage takes longer than a charging method which uses a constant current. Therefore, in a constant voltage charging method, it is important to make the charging time short as short as possible.